Summary: J. Am. Chem. SOC.1988, 110,149-114 149
Bonding and Coupling of Cl Fragments on Metal Surfaces
Chong Zheng,t*tYitzhak Apeloig,I and Roald Hoffmann*t
Contributionfrom the Departments of Chemistry and Materials Science Center, Cornell
University, Ithaca, New York 14853, and Technion-Israel Institute of Technology, Haifa 32000,
Israel. Received September 3, 1986
Abstract: The bonding of CH3,CH,, and CH fragments to Ti(0001), Cr(1lo), and Co(OOO1) metal surfaces is examined
with extended Hiickel band calculationson twedimensional slabsof metal and adsorbate. A local chemicalviewpoint is sought
through fragment analyses, decompositions of the density of states, and overlap population studies. All fragments tend to
restore their missing C-H bondswhen bound to these surfaces-CH3 prefers the on-top, CH2the bridging, and CH the capping
geometry. CH, anchors more stronglyto the on-top site of a metal surfaceof higher d band fillingsincethe antibondingfeature
at the top of the d band destabilizes sites of higher coordination. Similar conclusions hold for other fragments. Thus, the
mobility of these fragments is reduced on metal surfaces of higher d band filling. The mobility patterns of CH3,CH2,and
CH are examined. In general, on the way to products there are barriers to migration on the surface, a proximity or crowding
effect which makes it costly for two fragments to approach on the surface, a barrier, small or large, to the reaction with each
other, and finally a desorption barrier. When two C1fragments couple, the C-C us orbital rises from below the Fermi level.
It is initially filled and then empties as the reaction proceeds. Hence the lower the Fermi level (for metals at the right side
of the transition series) the smaller the reaction barrier. The theoretically expected decrease of the mobility of the organic
fragments on one hand and the higher coupling rate on the other, as the metal is changed from the left to the right side in
the Periodic Table, may be two of many reasons that are responsible for metals in the middle of the transition series having